The aim of the study was to investigate the reasons for, and incidence of, tooth mortality over a 10-year period in mentally retarded adults receiving regular dental care. The number of teeth present in 115 individuals (mean age in 1984 was 41.0, range 19-83 years) was registered in 1984 and 1994. The reasons for tooth mortality, medication utilization, frequency of dental care visits and cooperation during dental treatment were registered and related to tooth loss. The average incidence of tooth mortality was 3.72 teeth during the 10-year period. The mean number of dental care visits per year was 6.6. Most of the 428 teeth (58%) were lost due to periodontal disease. The preventive dental care given was not sufficient to arrest oral diseases. The data indicate, however, that achievement of cooperation in dental care situations not only makes dental treatment possible, but also leads to a decreased incidence of tooth mortality.